Primitive men already knew comets. The brightest ones look great and don't look like any other object in the sky.
Comets look like spots of light, often blurred, that leave a trail or hair. This makes them attractive and surrounds them with magic and mystery. Comets are fragile and small bodies, irregularly shaped, formed by a mixture of hard substances and frozen gases.
A comet consists of a nucleus, usually composed of ice and rock, surrounded by a hazy atmosphere called hair or coma. The American astronomer Fred Whipple described in 1949 the nucleus, which contains almost the entire mass of the comet, as a "dirty snowball" composed of a mixture of ice and dust.
Most of the gases that are expelled to form the hair are fragmentary or radical molecules of the most common elements in space: hydrogen, carbon, nitrogen and oxygen.
The head of a comet, including its diffuse hair, may be larger than the planet Jupiter. However, the solid part of most comets has a volume of a few cubic kilometers only. For example, the nucleus obscured by the dust of Halley's comet is approximately 15 by 4 kilometers in size.
The orbits of comets deviate considerably from those provided by Newton's laws. This may be because the escape of gases produces a jet propulsion that slightly displaces the nucleus of a comet out of its path.
Short-period comets, observed along many orbits, tend to fade over time as might be expected. Finally, the existence of groups of comets shows that the cometary nuclei are solid units.
In general, the orbit of comets is much longer than that of planets. On one end they can be brought closer to the Sun and, on the other, away from the orbit of Pluto.
When the comets approach the Sun and get hot, the gases evaporate, give off solid particles and form the hair. When they turn away again, they cool, the gases freeze and the tail disappears.
In each pass they lose matter. Finally, only the rocky core remains. It is believed that there are asteroids that are bare nuclei of comets.
The photo is the comet Kohouotek, which passed near Earth in January 1974. It had been detected far away, when it was going through the orbit of Jupiter.
There are comets with short orbital periods and others long. There are some that never exceed the orbit of Jupiter and others that go very far, until they leave the Solar System and no longer return.
Comet Encke, of short orbit, approaches every three years and three months. You can only see with a good telescope. Instead, the famous Halley's comet, which visits us every 76 years, and the Rigollet, which does it every 156 years, are still bright.
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